This application claims the priority of Application No. 100 397 82.4, filed Aug. 16, 2000, in Germany, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and apparatus for controlling the yaw dynamics and lateral dynamics in a road vehicle having one steering device each for the front axle and for the rear axle, and having electrically drivable δv and δh steer-angle actuators. These actuators are assigned to the axles individually and can each be driven via a controller. These controllers generate from desired/actual value comparisons of variables which are characteristic of the yaw-dynamic and the lateral-dynamic behavior of the vehicle (e.g., the yaw velocity {dot over (Ψ)} and a sideslip angle β) drive signals required for correcting the controlled variables, i.e., for the steer angle actuators. Control loops, provided for setting the steer angles δv and δh, are decoupled from one another. The desired value prescription signals, required for the two control loops, for the control parameters are generated by means of a reference model, implemented by an electronic computer, from processing at least one output signal, representing the driver's wish, from a steering element position sensor and a sensor output signal characteristic of the operating state of the vehicle, for example a speed sensor.
In vehicles that are equipped with steer-angle actuators that can be driven independently of one another for front axle steering and rear axle steering, it is possible in principle to obtain “extreme” vehicle movements that cannot occur in the case of a normal vehicle which can be steered only via the front wheels. For example, a sideslip of the vehicle, that is to say a movement of the vehicle obliquely relative to the vehicle longitudinal axis, is possible without the vehicle yawing (e.g., by virtue of the fact that the front axle steering and the rear axle steering are set to the same steer angle with reference to the vehicle longitudinal axis). It is also possible to obtain a yawing, that is to say a rotary movement of the vehicle about its vertical axis, without the vehicle executing a slipping movement.
The use of such vehicle movements, which can be obtained only with two-axle steering, should be reserved for reasons of safety for such driving situations in which the driver consciously adopts such an unaccustomed vehicle behavior, for example, maneuvering in a very tight space. Such vehicle movements should not be used in the “normal” operation of the vehicle, corresponding to the statistically dominant driving situations, for which operation the driver “customarily” expects a reaction of the vehicle corresponding to the driver's wish.
It is, therefore, an object of the invention to specify a method of the type disclosed herein, which upon actuation of a steering element provided for setting a driver's wish, for example a steering wheel or joystick, leads to a vehicle reaction which is largely analogous to that of a vehicle which has only front axle steering, but yet permits improved utilization of the lateral guiding forces that can be built up by the two steer-angle actuators.
This object is achieved in the case of a method of the type disclosed herein by the overall combination of controlling the yaw dynamics and lateral dynamics in a road vehicle having one steering device each for the front axle and for the rear axle, and having electrically drivable δv and δh steer angle actuators, as regards the basic idea, with determining the desired value Svsoll of the lateral force to be built up at the front axle is determined in a control process in accordance with a controller law of the form       S    vsoll    =                              l          h                ·        m        ·                  v          x                    L        ·          [                        Ψ          .                -                              β            .                    vsoll                +                              k            2                    ·                      (                                          β                v                            -              β              ⁢                              -                vsoll                                      )                              ]      or the desired value Svsoll of the lateral force to be built up at the front axle is determined in a control process in accordance with a controller law of the form       S    vsoll    =                              l          h                ·        m        ·                  a          y                    L        +                            J          z                L            ·                        [                                                    Ψ                .                            soll                        +                                          k                3                            ·                              (                                                      Ψ                    .                                    -                                                            Ψ                      .                                        soll                                                  )                                              ]                .            
In this case, the type of determination of a desired value of the lateral force at the front wheels provided in accordance with the desired value Svsoll of the lateral force to be built up at the front axle corresponds to a sideslip angle control at the front axle in the way provided in general for determining the desired value of the lateral force at the rear axle, while the type of determination of a desired value of the lateral guiding force at the front axle corresponds to a control of yaw velocity via the steer angle control loop assigned to the front axle. The approximate determination of the method of determining the desired values of the slip angle of the front wheels and the rear wheels of the vehicle is sufficient in the majority of statistically significant driving situations to be able to carry out a determination of steer angle for the front and rear wheels of the vehicle that is adequate for the situation.
In the case of a control device for a road vehicle having one steering device each for the front axle and for the rear axle and having electrically drivable steer angle actuators, assigned to said axles individually, for two mutually decoupled control loops which are suitable for implementing the control of lateral force based on yaw computation, the method of a lateral acceleration sensor which directly detects the lateral acceleration active at the center of gravity of the vehicle is particularly expedient.
Taking account of the vehicle geometry, it is also possible for this device to provide two lateral acceleration sensors whose spacing from one another measured in the longitudinal direction of the vehicle may be as large as possible.
Both owing to an ability to switch over the control device to various defined control modes, and by means of a specific selection between different reference model variants of the vehicle which are provided for generating the prescribed desired values for the front axle and rear axle steer angles δv and δh and implemented by a computer, it is possible to set the vehicle to correspondingly different types of its response behavior to an actuation, acting as an expression of a specific driver's wish, of a steering element, i.e., the vehicle type (sports car or heavy limousine) can be selected, which corresponds to the desired driving behavior of the vehicle. The control modes described herein may also be used whenever the rear axle steering is implemented by virtue of the fact that the rear wheel brakes can be driven individually to develop defined braking forces, as a result of which they can specifically influence the yaw behavior of the vehicle via the rear wheels even without a steer-angle actuator for the rear axle.
The automatic switchover of the control device to a control mode with the yaw velocity as a controlled variable in which the vehicle is moving in the extreme range of lateral dynamics, i.e., the lateral forces may no longer be increased by enlarging slip angles, results in the fact that the vehicle still remains capable of being effectively controlled even an extreme range and/or in the event of failure of the rear axle steering, and a high measure of safety is achieved to this extent.
A significant improvement in the quality of control is achieved by means of disturbance estimators assigned to the controlled variables, preferably ones whose design model corresponds to that of the controller for the observed controlled variable, since, by contrast with a controller with an integral-action component, it is not the control error that is integrated, but the error between measurement and estimate, which can then be used to compensate disturbances.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.